Many plants are adapted to just one phase of a forest succession. They suit either the first (pioneer) phase, or the mature forest phase, or somewhere in between. As a succession proceeds, some plant and soil trends are fairly universal.
During a succession, plant height increases, soil builds up and soil nutrients increase. Plants change:
- from species that regularly produce many small, light seeds, to those that produce a few large seeds, or occasionally produce many seeds
- from species with short life cycles, to long-lived trees
- from shade-intolerant species, to those that tolerate shade.
Plants on bare ground
On volcanic surfaces and exposed glacial moraines there is a common sequence. Lichens, mosses and small herbaceous plants appear first, soon followed by nitrogen-fixing shrubs.
All living things need nitrogen to grow. When a primary succession starts, little or no nitrogen is available from the surface, which is often ash, or gravel. Some plants are able to ‘fix’ nitrogen – bacteria in their roots change nitrogen from the air into a useable form. This allows them to live on nutrient-poor surfaces. Over time, the dead leaves and roots of these plants mix into the soil. The nitrogen content increases, so other plants are able to grow there too.
Native nitrogen-fixers include tree tutu, native brooms, blue-green algae, some lichens, kōwhai (Sophora species) and matagouri (Discaria toumatou). Introduced nitrogen-fixers include clovers (Trifolium species), tree lupin (Lupinus arboreus), Scotch broom (Cytisus scoparius), and gorse (Ulex europaeus). These plants all appear early in primary successions.